Filters are known to provide attenuation of signals having frequencies outside of a particular frequency range and little attenuation to signals having frequencies within the particular frequency range of interest. As is also known, these filters may be fabricated from ceramic materials having one or more transverse-electromagnetic-mode (hereafter "TEM-mode") resonators coupled together and formed therein. TEM-mode means that the electric and magnetic fields are in a direction that is perpendicular to the direction of wave propagation in a filter block. A ceramic filter may be constructed to provide a lowpass filter, a bandpass filter, a bandstop filter, or a highpass filter, for example.
FIG. 1 shows a representative prior art ceramic monolithic block filter 100. This filter contains a series of resonators 102 which extend from a top surface 104 to a bottom surface 106 of the block. The resonators 102 are capacitively coupled to an input pad 108 and an output pad 110. All external surfaces of the filter 100 are substantially covered with a conductive metallization coating with an exception of an area 112 surrounding the input pad 108 and the output pad 110 as well as an area 114 surrounding the resonators 102 on the top surface 104 of the filter block 100. It is notable that the metallization layer provides a substantially encapsulated casing for the energy which flows through the filter block 100.
At the design passband, the filter structure dominantly supports TEM-mode waves. Hence, the filter properties can be well predicted using the theory of TEM guided waves and telegrapher's equations relating to transmission line theory during modeling operations. However, away from the design passband, all filters have more or less pronounced parasitic passbands and other regions of poor attenuation. These problematic parasitic passbands are usually more obvious above the design passband, but they may also be present below the design passband. From a practical standpoint, these parasitic passbands may cause particular problems if they coincide with the 2nd and 3rd harmonics of the fundamental transmitter frequency, as strong harmonics of the transmitter frequency may be fed into the antenna.
The potential problems presented by these unwanted parasitic passbands cannot be understated. Oftentimes, these passbands will result in interference or unwanted noise in the signal. If the interference is sufficiently strong, it may result in the telephone call in the cellular system being dropped. Additionally, the transmission of harmonics at higher frequencies may create issues for a telecommunications provider which may have to be dealt with by the Federal Communication Commission (FCC).
Consequently, many designers of systems such as cellular telephones need additional attenuation over that provided by traditional ceramic monolithic block filters. To address this problem, designers oftentimes place a second lowpass filter in-line to suppress unwanted harmonic responses. This solution, unfortunately, is both expensive and time consuming, and may significantly add to the cost weight, and part-count of a completed product such as a cellular telephone, pager, or other electronic signal processing apparatus.
Another solution to the problem of unwanted parasitic passbands is to add lumped components to the printed circuit board, thereby creating an additional filter assembly which properly couples to the original filter and eliminates the unwanted higher frequencies. This solution is also expensive, labor intensive, and time consuming.
A ceramic filter design which addresses the problem of harmonic response suppression by attenuating the unwanted passbands through the introduction of a strategically positioned resistive paste deposit on an exterior surface of the dielectric block of ceramic without the addition of a second filter or lumped elements may result in a substantial savings in both space and cost. A ceramic transverse-electromagnetic-mode filter having a resistive paste design which suppresses unwanted parasitic passbands and spurious modes would be considered an improvement in the art .